Crosslinkable Fluorinated Polymer-Coated Self-Extinguishable Separators with Highly Enhanced Thermal Stability and Electrolyte Affinity for High-Performance Lithium-Ion Batteries

Thermal runaway is of great concern for developing high-performance Li-ion batteries (LIBs) and is accelerated by melting a separator at the elevated temperature during the battery failure. Here, we report a facile polymeric coating method for polypropylene (PP)-based separators (PPS) that enhances thermal, mechanical, and electrochemical performances. The conceptually designed double bond-contained polyvinylidene fluoride terpolymer (DPVDF) was coated on PPS (DPS) employing a simple dip-coating method and followed by the cross-linking process for DPVDF (DPSX). The modulus of DPSX increased from 49.06 MPa (PPS) to 64.29 MPa. The thermal shrinkage area of DPSX decreased less than 10% at 140oC for 30 min, whereas PPS showed 31.2% of shrinkage. High-temperature open-circuit voltage tests (140oC) showed that LiFePO4 (LFP) half-cells with PPS and DPS were short-circuited after 10 and 40 min, respectively. However, the LFP cell with DPSX showed no voltage drops even after 2 h. Furthermore, electrolyte wetting was significantly enhanced at DPS and DPSX compared to PPS. Moreover, electrolyte uptake was also increased from 65.25% (PPS) to 196.63% (DPSX), which is ascribed to the enhanced wettability and pore structure. The long-term cycling and rate capability tests of the LFP half-cells assembled with DPS and DPSX are significantly higher than the LFP half-cell with PPS, which is closely related to the reduced cell resistance due to the interfacial affinity of DPSX. Lastly, it was found that DPVDF coating granted self-extinguishing functions to the separator. Flame-test displayed that DPS and DPSX were instantly extinguished within a second and remained more than 74.89% in their weight after burning tests, while PPS lost more than 43.46% (dried) and 59.65 % (wetted) in their weight.